This invention relates to a method for generating channel audio signals for generating multi-channel audio signals employed for recording on a recording medium or transmission over a transmission channel in a stereophonic system or a multi-sound system for a motion picture film projecting system, video tape recorder or a video disc player.
The recent tendency in audio equipment is towards signal digitalization and a digital audio system has become popular not only as professional equipment but also in consumer equipment. Above all, the multi-channel system is now in widespread use in digital audio equipment, for example, in equipment handling 8- channel digital audio signals.
Among the equipment handling the eight-channel digital audio signals, there is, for example, a motion picture film projection system. In addition, audio signals or speech signals of plural channels, such as four to eight channels, are handled preferentially in a stereophonic or multi-sound system of a high- definition television, video tape recorder or a video disc player.
Above all, in the motion picture film projection system handling such eight-channel audio signals, it is a customary practice to record digital audio signals of, for example, left, left center, center, right center, right, surround left, surround right and sub-woofer channels, thus totalling eight channels, on the motion picture film. However, when recording the digital audio signals of the eight channels, since it is difficult to provide an area on the motion picture film in which to record as many as eight channels of the linear quantized 16-bit digital audio data, with the sampling frequency being 44.1 kHz, as employed in the so-called compact disc (CD), the audio data is compressed before recording. The technique of recording plural channel audio data as compressed digital patterns on the digital pattern recording region of the motion picture film is described in the present Assignee's co-pending U.S. patent Application Ser. No. 158,767 (European Patent Application published as No. 0600677A1 on Jun. 8, 1994).
The above-mentioned eight channels, thus recorded on the motion picture film, are associated with left, left center, center, right center, right and sub-woofer speakers, arranged on the side of a screen, and surround left and surround right speakers, arranged on the left and right sides of the spectator's seats. On the screen, there is projected a picture reproduced from the picture recording region of the motion picture film by a projector.
The multi-channel digital audio equipment is being widespread for consumer use is developed with existing media.
In order for the multi-channel digital audio equipment to be widespread in consumer use, it is desirable that multi-channel digital audio data be reproduced in existing or prospective channel systems, such as monaural, stereo or so- called 3-2 systems, in addition to an environment capable of reproducing all of the multiple channels. This means that reliable channel down-conversion is required for effective reproduction of data having three or more channels in a stereophonic sound field.
If, for example, recording data is not compressed data and can be outputted without decoding or the like operations, it is possible to down-convert the number of channels by combining plural channels.
However, if the eight channel data is to be reproduced with the monaural system, at least seven combining operations are required for down-converting the eight channels to a sole channel, thus resulting in an inefficient operation and in an complicated processing circuit.
On the other hand, with the multi-channel audio signals, the tendency is to compress data prior to recording. Such data compression affects the number of decoders for expansion by the down-converting method. For down-converting from the eight channels to two stereo channels, the eight channel compressed signals are all expanded into non-compressed data and combined among plural channels. Thus, eight decoders are required for two-channel stereo reproduction.
For monaural reproduction, many decoders are also required, which is equivalent to that for the eight channels, meaning inefficiency in the device construction. The playback system for down-converting the multichannel audio channels to a stereo or monaural channel is expensive and hence is unsuitable for consumer devices.